The present disclosure relates to a separator for a secondary cell interposed between an anode and a cathode, and a method of manufacturing the same.
Lithium ion secondary batteries commonly include, for example, anodes having composite lithium oxides, cathodes having materials capable of the absorption and emission of lithium ions, as well as separators and non-aqueous electrolyte solutions interposed between the anodes and the cathodes. Such anodes and cathodes are stacked with separators interposed therebetween, or are wound, after being stacked, to form winding electrodes having a columnar shape.
Separators function to electrically insulate anodes from cathodes, and to support non-aqueous electrolyte solutions. Microporous polyolefin membranes are commonly used to form separators for such lithium ion secondary batteries. Microporous polyolefin membranes have excellent electrical insulating properties and ion transmitting properties, and are widely used in separators for lithium ion secondary batteries or condensers.
Since lithium ion secondary batteries have a high output density and a high capacity density, but include non-aqueous electrolyte solutions, for example, organic solvents, the non-aqueous electrolyte solutions are decomposed by heat produced in an abnormal state, such as a short circuit or overcharging, and in the worst case, may ignite. In order to address these issues, lithium ion secondary batteries have several safety functions, one such safety function being a separator shutdown function.
The separator shutdown function is provided such that the micropores of separators are occluded by thermally molten resin materials when lithium ion secondary batteries generate abnormal amounts of heat, so as to suppress ionic conduction in non-aqueous electrolyte solutions, thereby stopping the progress of electrochemical reactions.
In general, it is known that if a shutdown temperature is low, the safety of lithium ion secondary batteries is high. One reason why polyethylene is used as a component of separators is that polyethylene has a moderate shutdown temperature. As such, separators, for example, uniaxially or biaxially stretched resin films, are used to porosify the separators and increase the strength thereof.
In recent years, demand for high capacity and high output secondary cells to improve the thermal stability of separators has increased. Lithium ion secondary batteries require high thermal stability in order to improve safety and increase capacity and output in a manufacturing process thereof and in use.
For example, when the thermal stability of separators decreases, interelectrode short circuits, due to damage or deformation of the separators caused by a temperature rise in the lithium ion secondary batteries, may occur, resulting in the overheating thereof or an increase in the risk of fire, due to membranes themselves being shrunk at general shutdown temperatures, so that anodes and cathodes may come into contact with each other. As a result, a secondary problem such as an internal short circuit may occur. Thus, there is a need to improve the safety of lithium ion secondary batteries by reducing the thermal contraction of separators by increasing the heat resistance properties of the separators.
For example, Japanese Patent Publication No. 2009-16279 discloses a separator having a coating layer in which microframeworks of a polyolefin-based resin material are coated on a glass layer. Japanese Patent No. 3797729 also discloses a separator for a battery in which an inorganic thin film is formed on the surface of a porous polyolefin film by a sol-gel process without charging empty pores.